Please watch Animation 8.5, The laser on your CD.
(Following paragraph is an excerpt from that animation.)
The key to the laser is that many atoms have one or more excited energy levels whose lifetimes are much longer (10-3 s) than those of most excited levels (10-8 s). Such relatively long-lived levels are called “metastable”. The first step in laser operation is to bring as many atoms as possible in an appropriate material to such metastable levels. (There are various ways to do this, depending on the material.) When one of the atoms in a metastable state spontaneously drops to a lower state, the photon that is emitted has just the right frequency to cause other atoms in metastable states to drop to the same lover state by emitting radiation of this frequency. The process is called induced emission and was predicted by Einstein in 1917. The result is an avalanche of photons, all of the same frequency and all coherent, which means that their electromagnetic waves are exactly in phase (in step) with one another.
Please go to this web site and go through how a laser works.
http://www.howstuffworks.com/laser.htm
You should go through at least :